1. Field of the Invention
The present invention relates to magneto-optical data recording and reproducing device wherein a data memory medium which applies modulation to a plane of polarization is subjected to the radiation of optical beams such as laser beams to store, reproduce or erase data.
2. Description of the Prior Arts
FIG. 6 is a schematic view of a conventional magneto-optical data reproducing device, disclosed in "Tricheps WS86 Chapt. V Optical Head and Data Reproducing Property (pp. 81-95) p. 93" by Y. ISHII, Sanyo Electric Motors, Ltd.
In FIG. 6, reference numeral 1 is a semi-conductor laser, 2 is a collimater lens, 3 is a beam splitter, 4 is an objective lens for forming a light-gathering part 5 is a data memory medium, 6 is a half wavelength plate, 7 and 8 are single lenses, 9 is a polarized beam splitter, 10 is a cylindrical lens, 11 and 12 are beam detectors, 21 and 22 are differential amplifiers, and 22 is an operational amplifier.
A divergent light beams with linear polarization emitted from the semiconductor laser 1 is converted into a parallel ray beam while passing through the collimater lens 2. The parallel ray beam then transmits through the beam splitter 3 and the objective lens 4 to the data memory medium 5, to form a light spot thereon.
A light beams reflected from the data memory medium 5, having passed through the objective lens 4, is partially reflected by and transmitted through the beam splitter 3. The reflected beam from the data memory medium 5 is transmitted through the half wavelength plate 6, whereby its plane of polarization is rotated by an angle of 45 degrees. Thereafter, the beams having passed through the single lenses 7, 8 are split into two pencil light beams with linear polarization each crossing the other at right angles by the polarized beam splitter 9. Herein, two of the single lenses 7, 8 for condensing the light beams are installed to constitute a telefocus unit.
The light beam reflected by the polarized beam splitter 9 is gathered by the cylindrical lens 10 and applied to the photo detector 11. On the other hand, the light beam having passed through the polarized beam splitter 9 is applied to the other photo detector 12.
Data retrieval is performed as follows. Since the reflected flux of the semiconductor laser beam with linear polarization applied to the data memory medium 5 has a plane of wave polarization rotated in response to the direction of a magnetic field by the Kerr effect, outputs from the photo detectors 11, 12 are changed according to the direction of the magnetic field. Herein, a differential detecting unit comprising the differential amplifier 21 for detecting a difference between the output of the photo detector 11 and the output of the photo detector 12, i.e. output of the operational amplifier 22, is disposed to eliminate noise from the data memory medium 5. Thus, data is accurately reproduced.
As for the detection of an error signal in this constitution, a focus error detecting unit according to an astigmatism method comprises the signal lenses 7, 8, the cylindrical lens 10 and the photo detector 11, while a tracking error detecting unit according to a push-pull method comprises the single lenses 7, 8 and the photo detector 12.
The tracking error detecting unit according to the push-pull method will now be explained in detail. The photo detector 12 to be used in the push-pull method is a two-piece photo detector. By locating the photo detector 12 at a position where light beams applied thereto are split into two equal pencil beams, the diffraction distribution of a light beam by the edge of a guide channel in the track of the data memory medium 5 can be measured. In other words, differential amplifier 23 computes a difference between outputs from two light beam-receiving domains so as to obtain a tracking error signal.
FIG. 7 shows the details of the tracking error detecting unit disclosed in "Trichepps WS86 Chapt. V Optical Head and Data Reproducing Property (pp. 81-95) p. 85" by Y. ISHII, Sanyo Electric Motors, Ltd.
Focus error detection according to the astigmatism method will now be explained in detail. In the astigmatism method, the cylindrical lens 10 is located in an optical path, so as to detect a change in the configuration of a beam caused by the aberration of a focal point. When the data memory medium 5 is located at the focal point, the four-piece photo detector 11 is disposed in a manner such that the configuration of a beam on the four-piece photo detector 11 becomes a true circle. A focus error signal can be obtained by computing a difference between signals obtained from two (diagonally positioned) domains of four light 11. The focus error detection is concretely illustrated in FIG. 8 (see "Trichepps WS86 Chapt. V Optical Head and Data Reproducing Property (pp. 81-95) p. 84" by Y. ISHII, Sanyo Electric Motors, Ltd.)
These conventional magneto-optical data recording and reproducing devices have the disadvantage that the optical system is inevitably large in size, since there is used a beam detecting means, whereby a light beam are split into two pencil beams each crossing the other at right angles by a polarized beam splitter, each of the split pencil beams is respectively condensed and then applied to two photo detectors 11, 12. In addition, for push-pull detection, it is necessary to determine the position of the photo detector 12 while detecting an error signal, in a case where the device is to be adjusted on assembling. Also, when push-pull detection is performed with a relatively small light spot formed on the photo detector 12 from a light beam condensed by the single lenses 7, 8, a drift in a tracking error signal is likely to be caused by a change in temperature. This drift can be inhibited by performing the push-pull detection with a relatively large light spot formed on the photo detector 12. However, numerous disadvantages are derived thereby. For instance, the photo detector 12 itself becomes large in size. Also, a detector signal in a high frequency range can not be obtained.